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| Home > Publications database > Element-Selective Investigation of Femtosecond Spin Dynamics in Ni$_{x}$Pd$_{1-x}$ Magnetic Alloys using Extreme Ultraviolet Radiation |
| Book/Dissertation / PhD Thesis | FZJ-2019-04266 |
2019
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag
Jülich
ISBN: 978-3-95806-411-9
Please use a persistent id in citations: http://hdl.handle.net/2128/23056
Abstract: Since Beaurepaire et al. discovered optically induced ultrafast magnetization dynamics in Ni thin films in 1996, a variety of experimental and theoretical studies have been made to understand the underlying physical mechanism of the magnetization dynamics. Among the suggested mechanisms, spin-ip electron-phonon scattering described by the microscopic three-temperature model (M3TM)[3] and superdiffusive spin transport[4],[5] are currently considered to be major physical mechanisms contributing substantially to the ultrafast spin dynamics. It is believed that more than one mechanism is responsible for the dynamics. The analysis of magnetization dynamics in Ni$_{x}$Pd$_{1-x}$ alloys with varying composition presented in this thesis provides deeper insight into the relation between the spin dynamics and spin-orbit coupling (SOC) in these materials. Our measurements were mostly done in the transversal magneto-optic Kerr effect (T-MOKE) geometry. To measure ultrafast magnetization dynamics with element selectivity, femtosecond pulses in the extreme ultraviolet (XUV) regime have been produced by laser-based high-order harmonic generation (HHG) in a noble gas. The XUV light from neon gas with an energy range from 20 to 72 eV covering the M$_{2,3}$-edges of ferromagnetic materials (52.7 eV for Fe, 58.9 eV for Co, and 68, 66.2 eV for Ni) can be expanded to form a spectrum, by a optical grating introduced on the sample surface. The magnetic contrast and the optically-induced dynamic response can be obtained using XUV light with energy corresponding to the absorption edges of the involved elements. In the Ni$_{x}$Pd$_{1-x}$ alloys, the intrinsically paramagnetic Pd is expected to be magnetically coupled to ferromagnetic Ni via complex exchange paths. One can study spin dynamics in the paramagnetic material (Pd) with induced magnetic moment as a consequence of the exchange coupling with ferromagnetic material (Ni). In addition, element selective measurements allow investigation of questions related to a possible superdiffusive spin transport between the Ni and Pd subsystems. The influence of the increased SOC on the spin dynamics in the Ni$_{x}$Pd$_{1-x}$ alloys can be also investigated by varying the mixing ratio of Ni and Pd in the alloy system. From static T-MOKE, we experimentally confirmed that increasing the Pd content in the alloy gives rise to a pronounced magnetic asymmetry of a bipolar shape at the Pd N$_{3}$-absorption edge (50.9 eV). Varying the mixing ratio in the Ni$_{x}$Pd$_{1-x}$ alloy changes the magnetic properties such as the magnetic moment $\mu$, the Curie temperature T$_{C}$, and the spin-flip scattering probability a$_{sf}$ of the alloy. We show that these parameters are closely related to the magnetization dynamics. Especially, the spin-flip scattering probability a$_{sf}$ scales with the atomic number Z as a$_{sf} \varpropto Z^{4}$ due to SOC. The magnetization dynamics in the Ni$_{x}$Pd$_{1-x}$ alloys can be tuned by the alloy stoichiometry due to a considerable difference in atomic numbers of Ni (Z$_{Ni}$=28) and Pd (Z$_{Pd}$=46). Our results can then be explained by considering the spin-flip scattering probability a$_{sf}$ within the framework of M3TM, and point to the crucial role of the Pd-mediated SOC in optically-induced spin dynamics in the Ni$_{x}$Pd$_{1-x}$ alloys.
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